Honeycomb structure and method for manufacture thereof

ABSTRACT

A honeycomb structure made of a silicon carbide-based porous body and having a number of through-holes extending in the axial direction, separated by partition walls. The strength and Young&#39;s modulus of the silicon carbide-based porous body satisfy the following relation:  
     Strength (MPa)/Young&#39;s modulus (GPa)≧1.1. The honeycomb structure contains refractory particles such as silicon carbide particles and the like and yet can be produced at a relatively low firing temperature at a low cost, has a high strength and a high thermal shock resistance, and can be suitably used, for example, as a filter for purification of automobile exhaust gas by a treatment such as plugging of through-channel at its inlet or outlet, or as a catalyst carrier, even under a high SV condition.

TECHNICAL FIELD

[0001] The present invention relates to a honeycomb structure used in afilter for purification of automobile exhaust gas, a catalyst carrier,or the like, as well as to a process for production of such a honeycombstructure.

BACKGROUND ART

[0002] Porous honeycomb structures are in wide use as a filter forcapturing and removing the particulate substance present in adust-containing fluid (e.g. exhaust gas emitted from diesel engine), oras a catalyst carrier for loading thereon a catalyst component capableof purifying the harmful substances present in an exhaust gas. It isknown that as a material constituting such a honeycomb structure, thereare used refractory particles such as silicon carbide (SiC) particlesand the like.

[0003] As a specific technique related thereto, there is disclosed, in,for example, JP-A-6-182228, a porous, silicon carbide-based catalystcarrier of honeycomb structure, obtained by using, as a startingmaterial, a silicon carbide powder having a given specific surface areaand a given impurity content, molding the material into a desired shape,drying the molded material, and firing the resulting material at atemperature of 1,600 to 2,200° C.

[0004] Meanwhile, there are disclosed, in JP-A-61-26550, a process forproducing a vitrifying material-containing refractory product, whichcomprises adding a vitrifying material to an easily oxidizable materialor a refractory composition containing an easily oxidizable material,mixing, kneading and molding them together with a binder, andopen-firing the molded material in a furnace containing a non-oxidativeatmosphere; and, in JP-A-8-165171, a silicon carbide molded materialobtained by adding, to a silicon carbide powder, an organic binder andinorganic binders of clay mineral series, glass series and lithiumsilicate series and molding the resulting material.

[0005] Also, in JP-A-6-182228 is introduced a process for producing aconventional porous, silicon carbide-based sintered body, whichcomprises adding, to silicon carbide particles as an aggregate, a bindersuch as vitreous flux, clayey material or the like, molding them, andfiring the molded material at a temperature at which the binder melts.

[0006] Further, as to a high-temperature use ceramic filter produced bymolding refractory particles which consist of silica sand, a groundpottery, a metal oxide (e.g. Al₂O₃, TiO₂ or ZrO₂), silicon carbide,nitride, boride, other refractory material, or the like and which areadjusted to a given grain size, to a porous, bottomed cylindricalmaterial using a refractory binder such as water glass, frit, glaze orthe like, there are disclosed, in JP-B-61-13845 and JP-B-61-13846, thepreferred average particle diameter and particle size distribution ofrefractory particles, the preferred porosity, average pore diameter,pore volume and partition wall thickness of cylindrical material, etc.

[0007] In the sintering (necking between particles) caused by therecrystallization of silicon carbide powder per se, shown inJP-A-6-182228, the silicon carbide component vaporizes from the surfacesof silicon carbide particles and the vaporized silicon carbide componentcondenses at the contact areas (necks) between silicon carbideparticles; as a result, the necks grow and the particles are bonded toeach other. There are problems, however, that this method brings a highcost since a very high firing temperature is required to be employed inorder to vaporize silicon carbide, and that the yield after firing isreduced since a material of high thermal expansion coefficient isrequired to be fired at a high temperature as well.

[0008] The above process allows production of a porous body of highstrength; however, the porous body has a high Young's modulus which isderived from the physical properties possessed by the silicon carbideused as a raw material.

[0009] In general, coefficient (R) of thermal shock resistance is shownby the following formula (1). In the following formula, S is a fracturestrength, ν is a Poisson ration, E is a Young's modulus, and α is athermal expansion coefficient. ν and α are characteristic values of eachmaterial and are almost constant in each material; meanwhile, S and Evary greatly depending upon the porosity, fine structure, etc. of eachmaterial.

R=S(1−ν)/ Eα  (1)

[0010] As shown in the above formula, thermal shock resistance isdirectly proportional to strength but inversely proportional to Young'smodulus. Therefore, according to the process for producing a sinteredbody disclosed in JP-A-6-182228, a sintered body having a sufficientthermal shock resistance, while it has a high strength though, can notbe produced since Young's modulus becomes high.

[0011] Meanwhile, there is a problem that a localized heat generation iscaused by the low thermal conductivity of the filter in the case of thetechnique of bonding a silicon carbide powder (as a raw material) with avitreous material, shown in JP-A-61-26550 and JP-A-6-182228 wherein alow firing temperature of 1,000 to 1,400° C. is employed; if one triesto burn the particulates collected by and deposited on the filter forreactivation of the filter, in the case that the sintered body producedby the technique is used, for example, as a diesel particulate filter(DPF) for removing the particulates contained in the exhaust gas emittedfrom a diesel engine.

[0012] Further, the filter shown in JP-B-61-13845 and JP-B-61-13846 isporous but is a bottomed cylindrical material having a large partitionwall thickness of 5 to 20 mm; therefore, there is a problem that thefilter is not usable under the high space velocity (SV) condition like afilter for purification of automobile exhaust gas.

[0013] The present invention has been made in view of theabove-mentioned situation of the prior art, and is intended to provide ahoneycomb structure which contains refractory particles such as siliconcarbide particles and the like and yet can be produced at a relativelylow firing temperature at a low cost, which has a high strength and ahigh thermal shock resistance, and which can be suitably used, forexample, as a filter for purification of automobile exhaust gas by atreatment such as clogging of through-channel at its inlet or outlet, oras a catalyst carrier, even under a high SV condition; and a process forproducing such a honeycomb structure.

DISCLOSURE OF THE INVENTION

[0014] According to the present invention there is provided a honeycombstructure made of a silicon carbide-based porous body and having aplural number of through-channels extending in the axial direction,separated by partition walls, characterized in that the strength andYoung's modulus of the silicon carbide-based porous body satisfy thefollowing relation:

[0015] Strength (MPa)/Young's modulus (GPa)≧1.1.

[0016] In the present invention, the strength and Young's modulus of thesilicon carbide-based porous body preferably satisfy the followingrelation:

[0017] Strength (MPa)/Young's modulus (GPa)≧1.25.

[0018] Further in the present invention, the strength and Young'smodulus of the silicon carbide-based porous body preferably satisfy thefollowing relation. In the present invention, the silicon carbide-basedporous body preferably contains silicon carbide particles as aggregateand metallic silicon as a binder:

[0019] Strength (MPa)/Young's modulus (GPa)≧1.3.

[0020] According to the present invention, there is also provided aprocess for producing a honeycomb structure, which comprises addingmetallic silicon and an organic binder to raw material silicon carbideparticles, mixing and kneading them to obtain a readily formable puddle,molding the readily formable puddle into a honeycomb-shaped moldedmaterial, calcinating the molded material to remove the organic binderin the molded material, and firing the resulting material, characterizedin that the addition amount of the metallic silicon is 15 to 40% byweight based on the total amount of the raw material silicon carbideparticles and the metallic silicon.

[0021] In the present invention, the firing is preferably conducted in atemperature range of 1,400 to 1,600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a graph obtained by plotting residual strength andstrength at room temperature against a temperature difference ΔT (° C.)between electric furnace and water.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] The honeycomb structure of the present invention is constitutedby a silicon carbide-based porous body, and the silicon carbide-basedporous body is produced so as to have strength and a Young's modulussatisfying the following relation.

[0024] Strength (MPa)/Young's modulus (GPa)≧1.1

[0025] As mentioned previously, thermal shock resistance is inverselyproportional to Young's modulus; therefore, to reduce Young's modulusrelative to strength is important in order to obtain a honeycombstructure of high thermal shock resistance. When the above value is lessthan 1.1, the resulting honeycomb structure is low in thermal shockresistance and, when such a honeycomb structure is used, for example, asa diesel particular filter (DPF) for removing the particulates containedin an exhaust gas emitted from a diesel engine and when the particulatescollected by and deposited on the filter are burned for reactivation ofthe filter, a sharp temperature difference appears in the filter and maybreak the filter; therefore, such a value is not preferred. In thehoneycomb structure of the present invention, the ratio of strength andYoung's modulus, of the silicon carbide-based porous body constitutingthe honeycomb structure is set so as to satisfy the above formula;therefore, the present honeycomb structure shows an excellent thermalshock resistance.

[0026] It is preferred to set the ratio of strength and Young's modulus,of the silicon carbide-based porous body so as to satisfy the followingformula, because a better thermal shock resistance is obtained.

[0027] Strength (MPa)/Young's modulus (GPa)≧1.25

[0028] It is particularly preferred to set the ratio of strength andYoung's modulus, of the silicon carbide-based porous body so as tosatisfy the following formula, because a sufficient thermal shockresistance is obtained.

[0029] Strength (MPa)/Young's modulus (GPa)≧1.3

[0030] When the strength and Young's modulus of the siliconcarbide-based porous body are set so as to roughly satisfy the followingrelation, the honeycomb structure made of such a silicon carbide-basedporous body may be used with no problem in, for example, DPF.

[0031] Strength (MPa)/Young's modulus (GPa)≧4.0

[0032] In the above formula, a value exceeding 4.0 is desirable from thestandpoint of thermal shock resistance; however, since it means a lowYoung's modulus, the honeycomb structure constituted by such a siliconcarbide-based porous body may have strain therein and the straingradually grows in the long-term use of the honeycomb structure and mayincur the fracture thereof. Therefore, such a value is not preferred.

[0033] The silicon carbide-based porous body constituting the honeycombstructure of the present invention, preferably contains silicon carbideparticles as aggregate and metallic silicon as a binder for bondingbetween silicon carbide particles. Thereby, in its production, arelatively low sintering temperature can be used for sintering, and ahoneycomb structure having an excellent thermal shock resistance can beobtained without conducting firing at a very high temperature such asshown in JP-A-6-182227. As a result, the production cost can be kept lowand the yield can be made high.

[0034] The honeycomb structure of the present invention uses metallicsilicon for bonding between silicon carbide particles and accordinglyhas a high thermal conductivity as compared with conventional structuresusing a vitreous material for bonding between refractory particles;therefore, when it is used, for example, as a DPF and when theparticulates deposited on the filter are burned for reactivation of thefilter, there appears no local heat generation such as to damage thefilter. Further, the honeycomb structure of the present invention is nota bottomed cylindrical material of large wall thickness such as shown inJP-B-61-13845 or JP-B-61-13846, but a porous honeycomb structure;therefore, it can be used under a high SV condition, for example, as afilter for purification of automobile exhaust gas or as a catalystcarrier.

[0035] Next, description is made on the process for producing thehoneycomb structure of the present invention. In producing the honeycombstructure of the present invention, first, metallic silicon and anorganic binder are added to raw material refractory particles, and theyare mixed and kneaded according to an ordinary method to obtain areadily formable puddle.

[0036] As the refractory particles used in the present invention, thereare used silicon carbide particles in view of the thermal resistance,etc. in considering that the honeycomb structure to be produced is usedin, for example, a DPF which is often exposed to high temperatures whenthe particulates deposited on the DPF are burned. As other refractoryparticles used preferably, there can be mentioned oxides such as Al₂O₃,ZrO₂, Y₂O₃ and the like; carbides such as SiC and the like; nitridessuch as Si₃N₄, AlN and the like; mullite; and so forth. Raw materialsfor refractory particles (e.g. silicon carbide) and metallic siliconcontain small amounts of impurities such as Fe, Al, Ca and the like insome cases; however, they may be used per se or after purification bychemical treatment such as chemical washing or the like.

[0037] In order to obtain a silicon carbide-based porous body having apreferred strength-to-Young's modulus ratio such as mentioned above,there can be mentioned a method of using, as a binder, a material ofsmall Young's modulus such as metal or the like. In particular, metallicsilicon used in the honeycomb structure and production process thereof,of the present invention is an excellent binder in view of the thermalresistance, corrosion resistance, handleability, etc. However, since thestrength-to-Young's modulus ratio mentioned above has a closecorrelation with the fine structure of the silicon carbide-based porousbody obtained, use of metallic silicon alone is not sufficient and it isnecessary to optimize the microstructures of the silicon carbide-basedporous body which is determined by the particle diameters andcompositions of the raw materials used, the firing temperature employed,etc.

[0038] The metallic silicon melts during firing and wets the surfaces ofthe silicon carbide particles and acts so as to bond the particles witheach other. In the present process for producing a honeycomb structure,the appropriate addition amount of metallic silicon varies dependingupon the particle diameters and shape of silicon carbide particles, butneeds to be 13 to 35% by weight and is preferably 15 to 35% by weight,more preferably 18 to 32% by weight based on the total amount of siliconcarbide particles and metallic silicon.

[0039] An addition amount of metallic silicon of less than 15% by weightis not preferred because the low Young's modulus brought about by theuse of metallic silicon does not appear sufficiently. An addition amountof more than 40% by weight is not preferred, either, because theresulting honeycomb structure has a dense structure and gives a highYoung's modulus.

[0040] The readily formable puddle obtained is molded into a desiredhoneycomb shape by extrusion or the like. Then, the resulting moldedmaterial is calcinated (degreased) to remove the organic bindercontained therein, followed by firing. The calcination is preferablyconducted at temperatures lower than the temperature at which themetallic silicon melts. Specifically, the molded material may becalcinated by once keeping at a predetermined temperature of about 150to 700° C., or may be calcinated by using a small temperature elevationrate of 50° C./hr or less in a predetermined temperature range.

[0041] When the calcination is conducted by once keeping the moldedmaterial at a predetermined temperature, the predetermined temperaturemay be one temperature level or may be a plurality of temperature levelsdepending upon the kind and amount of the organic binder used; when themolded material is kept at a plurality of temperature levels, the timesof keeping at these temperature levels may be the same or different.When the calcination is conducted by using a small temperature elevationrate, the small temperature elevation rate may be used only in onetemperature range or in a plurality of temperature ranges; when thesmall temperature elevate rate is used in a plurality of temperatureranges, the temperature elevation rates in these temperature ranges maybe the same or different.

[0042] The atmosphere used in the calcination may be an oxidizingatmosphere. However, when the organic binder is contained in the moldedmaterial in a large amount, the organic binder may be burned violentlyby the action of oxygen during the calcination to incur the quicktemperature increase of the molded material; therefore, the calcinationis preferably conducted in an inert atmosphere such as N₂, Ar or thelike in order to prevent the abnormal temperature increase of the moldedmaterial. The prevention of the abnormal temperature increase isimportant when a raw material of large thermal expansion coefficient(low thermal shock resistance) is used. When the organic binder is usedin an amount of 20% by weight or more based on the main raw material (asa superaddition), the calcination is preferred to be conducted in theabove-mentioned inert atmosphere. Also when the refractory particles areSiC particles or those which may be oxidized at high temperatures, it ispreferred that the calcination is conducted in the inert atmosphere atleast at a temperature at which the oxidation begins and at highertemperatures to prevent the oxidation of the molded material.

[0043] The calcination and the subsequent firing may be conducted indifferent steps in one furnace or different furnaces. Or, they may beconducted in a continuous step in one furnace. The former operation ispreferred when the calcination and the firing are carried out indifferent atmospheres; however, the latter operation is preferred fromthe standpoints of total time of calcination and firing, furnaceoperating cost, etc.

[0044] In order to obtain a structure in which the refractory particlesare bonded by the metallic silicon, the metallic silicon must soften. Inthe present process for producing a honeycomb structure, the temperaturerange of the firing is preferably 1,400 to 1,600° C. The optimum firingtemperature is determined from the fine structure and propertiesrequired for the honeycomb structure produced; however, the firingtemperature is more preferably 1,450 to 1,600° C., further preferably1,450 to 1,550° C.

[0045] A firing temperature of lower than 1,400° C. is not preferredbecause the temperature is lower than the melting point (1,410° C.) ofmetallic silicon and no porous structure is obtainable. A firingtemperature of higher than 1,600° C. is not preferred, either, becauseno sufficient reduction in Young's modulus owing to use of metallicsilicon is obtainable.

[0046] Incidentally, the production process employing recrystallization,shown in the above-mentioned JP-A-6-182228 enables bonding betweensilicon carbide particles and produces a sintered body of high thermalconductivity; however, since sintering is allowed to take place byvaporization and condensation, as mentioned previously, and siliconcarbide is vaporized, a firing temperature higher than that used in thepresent production process is needed and firing at 1,800° C. or moresordinarily at 2,000° C. or more is necessary in order to obtain asilicon carbide sintered body which is usable practically.

[0047] The atmosphere used in the firing is preferably selecteddepending upon the kind of the refractory particles used. In the presentinvention, since silicon carbide particles are used as the refractoryparticles, oxidation there of at high temperatures is feared. Therefore,it is preferred to use a non-oxidizing atmosphere such as N₂, Ar or thelike at least at a temperature at which oxidation begins and at highertemperatures.

[0048] The present invention is described in more detail below by way ofExamples. However, the present invention is in no way restricted tothese Examples.

EXAMPLES 1 and 2

[0049] A raw material SiC powder having an average particle diametershown in Table 1 and a metallic Si powder having an average particlediameter of 4 μm were compounded so as to give a composition shown inTable 1. To 100 parts by weight of the resulting powder were added 6parts by weight of methyl cellulose as an organic binder, 2.5 parts byweight of a surfactant and 24 parts by weight of water, followed byuniform mixing and kneading to obtain a readily formable puddle. Thereadily formable puddle was molded, using an extruder, into a honeycombshape having an outer diameter of 45 mm, a length of 120 mm, a partitionwall thickness of 0.43 mm and a cell density of 100 cells/in² (16cells/cm²). The honeycomb molded material was calcinated for degreasing,in an oxidizing atmosphere at 550° C. for 3 hours and then fired in anon-oxidizing atmosphere at a temperature shown in Table 1 for 2 hours,to produce silicon carbide porous sintered bodies of honeycomb structure(Examples 1 and 2). Test pieces were cut out from the sintered bodies,and measured for average pore diameter and porosity using a mercuryporosimeter. Further, using a material tester, the test pieces werefurther measured for strength according to four-point bending strengthtest, and measured and calculated for Young's modulus (from the relationbetween load and displacement) according to static elastic modulus testmethod; and the results obtained are shown in Table 1. Furthermore, thecrystal phase was identified by X-ray diffraction, which confirmed thatthey were composed of SiC and Si.

COMPARATIVE EXAMPLE 1

[0050] A silicon carbide porous sintered body of honeycomb structure wasproduced in the same procedure as in Examples 1 and 2 except that nometallic Si powder as a raw material was used, and under therecrystallization conditions shown in Table 1 (Comparative Example 1).The sintered body was measured and calculated for properties in the sameprocedure as in Examples 1 and 2, and the results obtained are shown inTable 1. Further, the crystal phase was identified by X-ray diffraction,which confirmed that it was composed of SiC alone. TABLE 1 SiC Averageparticle Si/SiC Firing pore Young's Strength (MPa)/ diameter ratioTemperature diameter Porosity Strength modulus Young's modulus Process(μm) (wt. %) (° C.) (μm) (%) (MPa) (GPa) (GPa) ratio Example 1 Bondingby 30 20/80 1450 10 45 20 17 1.17 Metallic silicon Example 2 Bonding by30 30/70 1450 10 45 20 15 1.33 Metallic silicon Comparative Sintering by15  0/100 2300 10 45 40 38 1.05 Example 1 recrystal- lization Reaction

[0051] (Thermal shock resistance test (in-water rapid cooling test))

[0052] Samples (test pieces cut out from the sintered bodies of Examples1 and 2 and Comparative Example 1) were placed in an electric furnace ofpredetermined temperature and then put into water of room temperaturefor rapid cooling. Thereafter, the samples were measured for strengthaccording to four-point bending strength test.

[0053] The strength of sample before heating in electric furnace wastaken as “strength at room temperature”, and the strength of sampleafter rapid cooling was taken as “residual strength”; residualstrength/strength at room temperature was plotted against temperaturedifference ΔT (° C.) between electric furnace and water, and theresulting graph is shown in FIG. 1.

[0054] In Comparative Example 1, the strength begins to decrease whenthe temperature difference ΔT reaches 300° C.; in contrast, in each ofExamples 1 and 2, the strength begins to decrease when the temperaturedifference AT reaches 400° C. Thus, the excellent thermal shockresistance of the present invention could be confirmed. Further, whenExample 1 is compared with Example 2, Example 2 shows a smallerreduction in strength than Example 1 and was superior in thermal shockresistance.

Industrial Applicability

[0055] As described above, in the honeycomb structure of the presentinvention, the strength and Young's modulus of the silicon carbide-basedporous body constituting the honeycomb structure are set so as to have aparticular ratio; therefore, the present honeycomb structure has anexcellent thermal shock resistance as compared with honeycomb structuresproduced by conventional recrystallization. Further, the presenthoneycomb structure contains silicon carbide particles as refractoryparticles and yet can be produced at a relatively low firing (sintering)temperature; therefore, it can be produced at a low cost, shows a highyield, and can be provided inexpensively. Furthermore, the presenthoneycomb structure has not only an excellent thermal shock resistancebut also a high thermal conductivity; therefore, when used, for example,as a DPF and when the particulates deposited thereon are burned forfilter reactivation, there occurs no local heat generation such as todamage the filter. Moreover, being porous, the present honeycombstructure can be suitably used, for example, as a filter forpurification of automobile exhaust gas or a catalyst carrier, even undera high SV condition.

1. A honeycomb structure made of a silicon carbide-based porous body andhaving a plural number of through-channels extending in the axialdirection, separated by partition walls, characterized in that strengthand Young's modulus of silicon carbide-based porous body satisfy thefollowing relation: Strength (Mpa)/young's modulus (Gpa)≧1.1.
 2. Ahoneycomb structure according to claim 1, wherein the strength andYoung's modulus of the silicon carbide-based porous body satisfy thefollowing relation. Strength (MPa)/Young's modulus (GPa)≧1.25
 3. Ahoneycomb structure according to claim 1, wherein the strength andYoung's modulus of the silicon carbide-based porous body satisfy thefollowing relation. Strength (MPa)/Young's modulus (GPa)≧1.3
 4. Ahoneycomb structure according to any of claims 1 to 3, wherein thesilicon carbide-based porous body contains silicon carbide particles asan aggregate and metallic silicon as a binder.
 5. A process forproducing a honeycomb structure, which comprises adding metallic siliconand an organic binder to raw material silicon carbide particles, mixingand kneading them to obtain a readily formable puddle, molding thereadily formable puddle into a honeycomb-shaped molded material,calcinating the molded material to remove the organic binder in themolded material, and firing the resulting material, characterized inthat the addition amount of the metallic silicon is 15 to 40% by weightbased on the total amount of the raw material silicon carbide particlesand the metallic silicon.
 6. A process for producing a honeycombstructure according to claim 5, wherein the firing is conducted in atemperature range of 1,400 to 1,600° C.